# std::fill_n

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class OutputIt, class Size, class T > OutputIt fill_n( OutputIt first, Size count, const T& value ); |  | (constexpr since C++20) (until C++26)
template< class OutputIt, class Size, class T = typename std::iterator_traits <OutputIt>::value_type > constexpr OutputIt fill_n( OutputIt first, Size count, const T& value ); |  | (since C++26)
| (2) |
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value ); |  | (since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class Size, class T = typename std::iterator_traits <OutputIt>::value_type > ForwardIt fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value ); |  | (since C++26)

1) Assigns the given value to the first count elements in the range beginning at first if count > 0. Does nothing otherwise.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If any of the following conditions is satisfied, the program is ill-formed:

- value is not writable to first.
- `Size` is not convertible to an integral type.

### Parameters

first | - | the beginning of the range of elements to modify
count | - | number of elements to modify
value | - | the value to be assigned
policy | - | the execution policy to use
Type requirements
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.

### Return value

Iterator one past the last element assigned if count > 0, first otherwise.

### Complexity

Exactly std::max(0, count) assignments.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

fill_n
    template<class OutputIt, class Size,
             class T = typename std::iterator_traits<OutputIt>::value_type>
    OutputIt fill_n(OutputIt first, Size count, const T& value)
    {
        for (Size i = 0; i < count; i++)
            *first++ = value;
        return first;
    }

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1,2)

### Example

Run this code

    #include <algorithm>
    #include <complex>
    #include <iostream>
    #include <iterator>
    #include <vector>

    int main()
    {
        std::vector<int> v1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

        // replace values of the first 5 elements with -1
        std::fill_n(v1.begin(), 5, -1);

        std::copy_n(v1.cbegin(), v1.size(), std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';

        std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
        #ifdef __cpp_lib_algorithm_default_value_type
            std::fill_n(nums.begin(), 2, {4, 2});
        #else
            std::fill_n(nums.begin(), 2, std::complex<double>{4, 2});
        #endif
        std::copy_n(nums.cbegin(), nums.size(),
                    std::ostream_iterator<std::complex<double>>(std::cout, " "));
        std::cout << '\n';
    }

Output:

    -1 -1 -1 -1 -1 5 6 7 8 9
    (4,2) (4,2) (4,8)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 283 | C++98 | `T` was required to be CopyAssignable, but `T` is not always writable to `OutputIt` | required to be writable instead
LWG 426 | C++98 | the complexity requirement was “exactly count assignments”, which is broken if count is negative | no assignment if count is non-positive
LWG 865 | C++98 | the location of the first element following the filling range was not returned | returned

### See also

fill | copy-assigns the given value to every element in a range (function template)
ranges::fill_n (C++20) | assigns a value to a number of elements (algorithm function object)

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